A thermoplastic resin microporous membrane is widely used as a membrane for separation of substances, a membrane for selective permeation of substances, a membrane for isolation of substances and the like. Examples thereof include, for example, a battery separator to be used in lithium ion secondary battery, nickel-hydrogen battery, nickel-cadmium battery or polymer battery; a separator for electric double layer capacitors; various filters such as reverse osmosis filtration membrane, ultrafiltration membrane and microfiltration membrane; a moisture-permeable waterproof clothing; a medical material and the like.
In particular, a polyethylene-made microporous membrane is suitably used as a lithium ion secondary battery separator, the polyethylene-made microporous membrane ensuring ion permeability due to impregnation with an electrolytic solution, excellent electrical insulating properties, and a pore blocking function of avoiding an excessive temperature rise by cutting off a current at a temperature of approximately 120 to 150° C. when the temperature in a battery shows an abnormal rise. However, if the temperature rise in the battery continues for some reasons even after pore blocking, the polyethylene-made microporous membrane may shrink and rupture. This phenomenon is not limited to the polyethylene-made microporous membrane, but even in a microporous membrane using other thermoplastic resins, the phenomenon above cannot be avoided at a temperature not less than the melting point of the resin.
In particular, a lithium ion battery separator is deeply related to battery characteristics, battery productivity and battery safety and is required to be excellent in mechanical properties, electrode adhesion, ion permeability, dimension stability, pore-blocking property (shutdown property), melt rupture property (meltdown property) and the like.
Furthermore, in a wound battery, to enhance the volume energy density, it is desirable that an electrode body having a lamination of a negative electrode, a separator and a positive electrode can be packed in a container at a high density. Consequently, the separator will also be constrained by an increasing requirement not only for thickness reduction, but also for high-density winding.
In Example 1 of Japanese Patent No. 4988973, both surfaces of a polyethylene microporous membrane are coated with a coating solution prepared by dissolving a polyvinylidene fluoride-hexafluoropropylene copolymer in a mixed solvent of dimethylacetamide/tripropylene glycol, followed by putting in a coagulation bath, and subjecting to water washing and drying, thereby obtaining a non-aqueous secondary battery separator.
In Example 1 of Japanese Patent No. 5226744, a coating solution prepared by dissolving VdF/HFP/CTFE in a mixed solvent of DMAc/TPG is put in a tank in which two Mayer bars have been arranged in parallel at the bottom thereof, and a polypropylene microporous membrane is transported from the upper part of the tank into the tank at a transport rate of 3 m/min to pass through the two Mayer bars to coat the both surfaces with a fluorine-based solution, followed by putting it in a coagulation tank, and subjecting to water washing and drying, thereby obtaining a composite porous membrane.
In recent years, studies are being made on the use of a lithium ion secondary battery over a wide range including a large-sized tablet, a lawn mower, an electric bicycle, an electric vehicle, a hybrid vehicle, a small boat and the like. To this end, a large battery compared with conventional batteries used in a small electronic device such as portable phone or portable information device is required. Consequently, a separator incorporated into a battery is required to have the increased width.
On the other hand, in a separator obtained by arranging a porous layer on a microporous membrane, an increase in the width of the microporous membrane makes it difficult to arrange a porous layer with a uniform thickness in the width direction by coating. In particular, when a Mayer bar is used, deflection occurs in the Mayer bar itself as the coating width increases and, thus, uniform coating is difficult.
Non-uniform thickness of a porous layer means that a thin portion is partially found in the porous layer, and the average thickness of the porous layer is required to be a thickness as large as 1.5 to 2 times the minimum necessary thickness for sufficiently ensuring the function of the porous layer. This becomes a factor for increased cost due to increase in the resin amount. Furthermore, in an electrode body where a positive electrode and a negative electrode are laminated or wound via a separator, the increase in thickness of the separator decreases the number of laminations or windings in the electrode body, giving rise to a hindrance to a high capacity of a battery.
The non-uniform thickness of the porous layer also adversely affects the winding appearance of the separator roll, for example, by generating a streaky dent or a convex streak in the separator roll or by producing wavy wrinkles at the edge of the roll. In the future, it is predicted that the length of the separator increases to reduce replacing loss of materials in the production process of an electrode body and since an increase in the length involves an increase in the number of turns in a separator roll, the roll diameter becomes large. In turn, the problem with the winding appearance becomes more pronounced.
Under conventional coating techniques, a porous layer having a uniform thickness in the width direction can hardly be arranged on a wide microporous membrane, and the separator roll cannot be sufficiently satisfied in terms of winding appearance, leading to a decrease in the yield.
It could therefore be helpful to provide a battery separator capable of being wound at a high density and suitable for achieving a high battery capacity, wherein a porous layer having a uniform thickness and improving electrode adhesion is laminated on at least one surface of a polyolefin microporous membrane having a width of 100 mm or more and a variation range of an F25 value in the width direction of 1 MPa or less.